Conventional head assemblies for magnetic tape drives comprise separate heads for recording (writing) and playback (reading). Many assemblies employ a combination of write and read heads for a given channel arranged so that all of the heads can be aligned with a selected track on a magnetic recording tape. For example, a "read write read" head assembly may have three heads positioned along a common line so that a write head is positioned between two read heads. This configuration allows the head assembly to perform a "read after write" operation in which the read head following the write head verifies the integrity of the just-written data for either a forward or reverse direction of tape travel.
In state of the art high areal density recording, track pitch is extremely small. If either the head or the tape is even slightly misaligned with respect to the reference plane of the tape drive, the read head may pick up only a partial signal from the just-written track, or may even begin reading the signal from the next adjacent track. This off-track position may give rise to a very large perceived error rate during a read after write operation even if the actual recording was error-free.
Head-track misalignment has been previously addressed in a number of ways. For instance, mechanical tolerances on data cartridge construction may be tightened and initial alignment of the head/actuator assembly may be finely adjusted. With the advent of high density magnetic tapes, however, track widths have decreased to the point where incorporating the necessary mechanical precision into every drive and every cartridge becomes impractical and prohibitively expensive. Even when the cartridge and drive are manufactured according to close specifications, head-track misalignment may still occur. For example, error may result from the biasing or skewing of tape which travels at high speeds under tension in a data cartridge. This type of error is especially apparent when a new track is written on the tape or data is overwritten onto an existing track. When the existing track was created by a different drive from the one in use, the opportunity for error is even greater.
Though improper tape alignment with respect to the reference plane is, in theory, a separate circumstance from improper head alignment, in practice they produce common misalignment effects.
Automatic techniques for reducing or compensating for these effects are known in the magnetic recording industry. For example, instrument recorders having a fixed, multitrack head assembly have previously employed a servo control method to compensate for head alignment error occurring as a magnetic tape is wound from reel-to-reel in the recorder. Automatic rotational adjustment of the head was controlled using timing signals generated from two pre-recorded information tracks spaced apart so as to be played back by the two outermost heads of the multitrack assembly. This technique was used to correct timing skew between tracks being read simultaneously across the width of the media.
Methods for correcting head position using transverse adjustment of the head assembly are also known for single track head assemblies. For example, U.S. Pat. No. 5,001,580 (Aranovsky et al.) reviews the geometry of head-track alignment errors and methods of compensation which have been incorporated into several previous magnetic recording systems. The method disclosed in that patent pertains to a recording system having a three channel head assembly with a write head and two read heads arranged along a common line, nominally parallel to the tape transport path. Each read head is positioned so as to sequentially read a pre-recorded data pattern oriented at a known slope with respect to the tape centerline. The angular offset between the common line of the two read heads and the centerline of the tape is then calculated based on timing signals derived from the read heads. Those signals enable a transverse adjustment of the head assembly to be made via a stepping process to position one of the two read heads over the track center for either forward or reverse reading. The offset calculation is performed once for a particular tape drive and can be saved within the drive for future adjustments.
IBM Technical Disclosure Bulletin, V. 17, No. 1, 1974 (Schwarz), describes a method for correcting the position of a head assembly comprising one write head and one read head. The method uses a small pre-recorded test sector within each track on a magnetic tape. Both heads, operated in a read mode, generate separate signals from the test sector on one track. The magnitude of the difference between the signals is indicative of the displacement between the two heads with respect to the track. The data is stored digitally and can be recalled to adjust the head assembly transversely so that the read head is centered over the track.
Various mechanisms are also known for controlling the transverse position of the head assembly with respect to a magnetic tape (also known as track finding). For example, U.S. Pat. No. 4,750,067 (Gerfast) discloses a head positioning mechanism for a multitrack data cartridge recorder. The mechanism utilizes a stepper motor to move the head incrementally across the width of a magnetic tape having a plurality of parallel tracks so as to locate the head over a selected track.
The QIC-3GB and QIC-10GB Development Standards further propose the use of a pre-recorded track on the tape and a servo loop mechanism in the drive to minimize the effects of the many sources of head-track error. Both read heads are used in a servo loop to provide transverse head position control.